SMALL ANGLE PION FLUX PRODUCED IN A THICK BERYLLIUM BY HIGH ENERGY ELECTRONS*

Transcription

1 SLAC-PUB-260 January 1967 SMALL ANGLE PION FLUX PRODUCED IN A THICK BERYLLIUM BY HIGH ENERGY ELECTRONS* TARGET A. Boyarski, F. Bulos, W. Busza, D. Coward, R. Diebold J. Litt, A. Mintenf B. Richter, and R. Taylor Stanford Linear Accelerator Center Stanford University, Stanford, California ABSTRACT The pion yield produced by 16-GeV electrons in a 0.6 radiation length beryllium target was studied at laboratory angles of 0.2 to 1. lo. The experimental yields are somewhat larger than those esti- mated using the Drell and p -diffraction models for pion photoproduc- tion, and they do not appear to decrease at small angles as expected from the theoretical models. (Submitted to Physical Review Letters) * Work supported by the U. S. Atomic Energy Commission. On leave from CERN

2 High energy electron beams produce useful beams of strongly interacting particles via bremsstrahlung and subsequent photoproduction in a thick target. Drell originally pointed out that y-ray pair production of strongly interacting particles with the interaction of one of these particles with the target nucleus could lead to large differential cross sections at small angles. It has also been found that the coherent photoproduction of p mesons from complex nuclei makes a large contribution to pion fluxes at high energy. 2. We have studied the pion yie1d.s from a 0.6 radiation length beryllium target and compared the results with a calculation by Tsai3 based on the Drell and p mechanisms. This letter describes the yields at laboratory angles s lo ; the two accompanying letters describe results obtained near 0 4 and at 2 and 3. 5 Electron beams of various energies between 10 and 16 GeV were directed into a 0.6 radiation length beryllium target placed between the end of the Stanford 20-GeV linear accelerator and the beam transport system normally used to carry the electron beam to the experimental area. Secondary particles produced in the target were carried about 300 meters by this transport system to the experi - mental area, and focused at the detector. Steering magnets located in front of the target allowed the angle of the incident beam relative to the transport system to be varied over the range from 0 to lo. The momentum acceptance of the system was set to values ranging from 1/8yC to 2%. The solid angle acceptance was measured directly by scanning the angular acceptance limits with the elec- tron beam; it was typically 5 X 10-8 steradian but varied with secondary particle momentum (h 15%), and time (-lr 10%). A sketch of the detection equipment is shown in Fig. 1. The shower counter consisted of 16 layers of l/4-inch plastic scintillator separated by l/4-inch lead plates, and had an energy resolution of rt5g for 10 GeV electrons. A coincidence -2 -

3 between the small scihtillators, Tl and T2, caused the SDS-9300 computer to read the range counters and the pulse height in the shower counter. This range versus pulse height distribution was the basis of the separation of electrons, pions, and muons. Good e-r separation was necessary since the e- to rr- ratio at the detectors was typically 4OO:l and 2O:l at 0.5 and lo, respectively. The number of pion events in the lo runs was obtained by comparing the shower counter pulse height spectrum for the 0.5 and lo runs, using the 0.5 runs with their much larger ratio of electrons to pions to estimate the electron contamination in the lo runs. For the 0.5 and smaller angle runs, the pion yields were obtained by considering only those events having less than GeV electron-equivalent pulse height in the shower counter and pulses in at least counters R, MO and Ml; this data should be quite free of electron contamination. The correction factor for having taken only these safe events was obtained by observing the fraction of pions at lo which satisfied the conditions; this fraction varied somewhat with momentum, but was typically 50%. The data were corrected for pile-up in the pulse height analyzer (typically less than 2%), interactions with material upstream of the shower counter (70/c), and pion decay in flight (a factor of at 10 GeV/c). No 7~ Kp separation was made. The long flight path reduces the K contam- ination to well under 1%; the antiproton rates are also under lyc of the 7~~ rate. Based on the results of the other beam survey experiments, 4,5 + our 7~ rates have a proton contamination of roughly 6%; no correction has been made for this. The experimental results are shown in Fig. 2. The errors shown on the points in Figs. 2a and 2b correspond to counting statistics and +80/o drift uncer- tainty in the beam monitor, f 10yC possible variation in A 51 for the different -3-

4 momenta, and * 10% uncertainty in the ne separation which may vary with mo - mentum. The normalization of the points is uncertain by &20% (solid angle and re separation). The errors shown on the points in Figs. 2c and 2d correspond only to statistics and + 8 o/c for the monitor, leaving an uncertainty in normaliza - tion of %25%; from the other effects. The curves marked Drell in the figures correspond to Drell s original formula with c (n Be) = 210 mb, averaged over the thick target shower. The curves marked p correspond to the coherent photoproduction of polarized p s with a laboratory cross section of da xi2 = 73 k2 A OA 2/3 t pb/sr where k is the photon energy in GeV and A = 9 for beryllium. The process is assumed to conserve helicity giving a sin2 8 decay distribution where 8 is the angle between the decay r in the p rest frame and the p momentum in the overall center-of-mass. 6 The curves marked Crossland were obtained from the Cocconi, Koester and Perkins (CKP) formula for yields from proton beams, but 7 with constants obtained from the CEA pion yields, At a pion momentum of 13 CeVhc the experimental points at 0.52 and 1.07 are in good agreement with both the Crossland and the Drell + p curves. At the lower momenta the experimental yields are larger than expected. The angular distribution for 7 GeV/c rf is shown in Fig. 2c; it is seen that the data does not fall off at small angles as expected from the Drell OPE mechanism. The excess pions at 7 GeV thus cannot simply be due to uncertainty in the strengths of the Drell and p mechanisms, but are presumably due to other mechanisms. 8 At the present time it is difficult to imagine individual mechanisms which can result in such a large increase of the yield; for example, a crude calculation estimating -4 -

5 the pions from the lower vertex of the Drell diagram indicates that this contribution is down from the Drell peak by a factor of 4. Data taken as a function of electron energy are shown in Fig. 2d and indicate that the energy dependence of the cross sections are quite consistent with that expected from the Drell and p mechanisms. WC are grateful for the dedicated work of R. B. Neal, V. Price, E. J. Seppi, 5. L. Warris and their staff in the construction and operation of the accelerator and experimental areas. Theoretical assistance from Drs. Tsai, Drell and Crossland is thankfully acknowledged

6 REFERENCES S. D. Drell, Phys. Rev. Letters 5, 278 (1960). L. J. Lanzerotti, R. B. Blumenthal, D. C. Ehn, W. L. Faissler, P.M. Joseph, F. M. Pipkin, J. K. Randolph, J. J. Russell, D. G. Stairs and J. Tenenbaum, Phys. Rev. Letters 15, 210 (1965) r 3. Y. S. Tsai, SLAC User s Handbook, Section D3 (1966). A. Barna, J. Cox, F. Martin, M. L. Perl, T. H. Tan, W. T. Toner and T. F. Zipf, Phys. Rev. Letters 18, (1967). S. M. Flatte, R. A. Gearhart, T. Hauser, J. J. Murray, S. G. Wojcicki, R. Morgado, M. Peters, P. R. Klein, and L. H. Johnston, Phys. Rev. Letters 18, (1967) These curves were computed by Van Whitis using computer programs he wrote using Tsails formulae. A. D. Crossland, SLAC- TN (1966, unpublished). which 8. Note that certain of the refinements suggested for the Drell mechanism can add to the small angle yields. In particular, the gauge invariant model of Stichel and Scholz, Nuovo Cimento 34, 1381 (1964), effectively adds on a correction which is fairly independent of angle and is roughly 600/C of the Drell peak. Caution must be exercised, however, since their calculations were for the specific case yp -+

7 I FIGURE CAPTIONS 1. Sketch of the detection system (not to scale). 2. (a) 7r- yields at 0.52 from 16-GeV electrons incident on 0.6 radiation lengths of beryllium. The uncertainties shown are those expected to fluctuate from point to point; in addition there is an uncertainty in normalization of k 20%. The curves are discussed in the text. (b) 7~~ yields at (C) 7 GeV/c x+ angular distribution. (d) 7 GeV/c 7r- excitation data.

8 N I

9 I I I I I I I I I 1 b) /, DREl r p,-(gevk) Fig. 2

10 IO -3 III1 1 III 11 IIll I II C Ee-= 16GeV I l- P T+= 7 GeV/c 2- ( ) 5 b--t as x -=I I- -1 I- dros= 1 DRELL+pm IO -3 0 II I I I I I I I I I I I I I. 0 IO T+ I I I I I I 1 2- x o7o cd) PlT- = 7 GeVk I,,L+p II CT I2 14 I6 Ee- (GeV) 682Al Fig. 2